In the construction industry, a water displacement test is used to establish the bulk material density associated with the acceptability of the material durability used to form the pavement or underlayment of roadways or other construction or building projects. For example, most roadways consist of a plurality of materials and layers including different types of aggregates, rocks, stones, gravel, or other materials which are compacted together to form the foundation and/or structure for the roadway surface and construction structures. These material compositions can be described as "compacted mixtures". The composition of the compacted mixture is generally considered to be an important factor in the service life of the construction project. In order to assure that the construction projects (such as a particular roadway or substructure) exhibit sufficient performance characteristics and useful service lives, most construction projects are constructed to certain minimum build specifications or standards. One important standard used to assess the acceptability of the compacted mixture, particularly in the asphalt and soil industries, is a bulk specific gravity and density measurement of the compacted mixture.
A typical standard test method used to assess the bulk density of the compacted bituminous (asphalt) mixture is ASTM D2726. During evaluation, a field sample or laboratory molded sample is obtained. The pavement specimens are usually taken from pavements in the field with a core drill, diamond or a carborundum saw, and the like. In any event, the core specimen, whether from the field or molded in the laboratory, is typically in the shape of a cylinder. As such, the field specimen typically exhibits a rough uneven exterior surface. In order to preserve the integrity of the core specimen during and after removal from pavements or molds (and during testing), care is taken to avoid distortion, bending, or cracking of the specimens.
Generally described, the ASTM D2726 test method involves measuring the specimen's weight, both in air and in water. More particularly, during this analysis, three different weights of the specimen are measured; a weight in water, a dry weight, and a saturated surface dry weight. The difference between the sample's weight in the air and in the water is equal to the weight of the water displaced (which can be measured, this determines the volume of the water displaced) and saturated surface dry weight can be used to ascertain the amount of water absorbed by the sample. Since the volume of displaced water is known, the specific gravity of the sample can be determined. The test method results can be used to determine the unit weight of compacted construction material (typically dense) mixtures. This method is generally accepted as being accurate for smooth and/or non-porous samples. Indeed, the method is used around the world to determine the conformance to various regulatory specifications, both for prepared laboratory samples and field extracted samples.
The ASTM D2726 test method is not recommended for use with samples that contain open or interconnecting voids or that absorb more than 2% of water by volume, or both, as determined by saturated surface dry weight, e.g., "porous samples". Using this test method for porous samples can provide unreliable density measurements. This is attributed to the variable amount of water absorbed by the porous sample which can result in an inaccurate volume determination and, thus, an inaccurate and unreliable density determination.
Presently, specification standards require that the porous samples be measured differently from the ASTM D2726 test method. Typically, ASTM D1188 is recommended for use if the percent water absorbed by the specimen or sample exceeds 2%. ASTM D1188 is directed to the use of "paraffin-coated specimens" to seal the sample to prevent water infiltration into the porous samples whose specific gravity is to be determined by water displacement methods. In one paraffin application, in order to coat the sample, the sample is submerged into hot-melted paraffin wax and pulled out and cooled allowing the paraffin to solidify and form a shell around the sample. The density is then determined using the water displacement method. Unfortunately, the thickness of the paraffin layer can be inconsistent, which can produce variability in the measurement results. Further, penetration of the wax into the voids themselves can result in inflated density measurements. In addition, paraffin wax is difficult, if not impossible, to completely remove once applied, and the specimen is generally rendered unsuitable for further analysis. Further, the presence of a supply of melted hot wax can introduce safety hazards for laboratory personnel.
ASTM D1188 describes using Parafilm.RTM., an elastomeric self-sealing moisture proof film obtainable from most scientific suppliers. As described, three pieces of Parafilm.RTM. are cut from the roll, two 100.times.100 mm (4.times.4 in) and one 100.times.200 mm (4.times.8 in). The backing is pulled off the backside of one of the 100.times.100 pieces and opposite sides of the film are grasped to stretch the film (carefully, without creating holes) and then placing the stretched film over one end of the specimen and pressing the sides of the stretched film around the sample. The specimen is turned over and positioned on a cushioned foam mat and the other end is wrapped with another piece of the stretched Parafilm.RTM.. Another specimen is used to force the air pockets from both surfaces by pressing against a piece of foam which is positioned on top of the wrapped specimen. A sharp knife is used to trim the excess film, keeping a minimum of 15 mm (0.5 in) on the side of the specimen at each end. The third piece of film is then applied. This elastomeric film method determines the "apparent specific gravity of Parafilm.RTM." by using the specific gravity of an aluminum calibration cylinder before and after it is wrapped with Parafilm.RTM. as noted above. Unfortunately, this sealing method is relatively labor intensive. Further, the amount of Parafilm.RTM. used during the wrap as well as how it is stretched over the sample can be inconsistent, which can result in measurement inconsistencies. In addition, air can be trapped under the film during the film wrapping process. Still further, the film is susceptible to puncture both during application and during actual testing potentially allowing water to enter through the puncture. Clearly, either the trapped air or puncture can adversely affect the reliability of this method.
Another method for measuring the bituminous mix density by water displacement with coated specimens is proposed by Jack E. Stephens, in a report entitled "Bituminous Mix Density by Coated Specimen," Project Number 67-5, Connecticut Department of Transportation (January 1973). This method proposes using a vacuum pump and two sheets of acetate to wrap the specimen. The first acetate or plastic sheet is apparently heated and held in tension in position while the sample is raised until it contacts the plastic. The rising specimen enters the center of the tensioned sheet and forces the plastic to wrap around the upper surface and the sides of the specimen. The tensioned material provides a surplus of material extending from both sides when the first sheet is released from the tensioning members (plastic clamps). This surplus material is then trimmed, leaving the lower surface and a minor portion of the sides exposed. The sample is then turned such that the lower surface faces upward and a second sheet of acetate is wrapped over the remaining exposed surface, and the second sheet overlaps a portion of the first sheet along the sides. Again, the excess material of the second acetate sheet is trimmed. After trimming, the sample is enclosed in a shrink-wrap acetate material layer with a double layer of the acetate formed along its sides. This method also proposes using a heater when forming the plastic over the specimen, to soften the plastic to facilitate the molding of the (heated) soft plastic to the sample. Unfortunately the heating temperature and time of the plastic can affect the degree of the softness of the plastic, which, in turn, affects the adherence of the plastic to coarse, irregular, and porous samples, thereby undesirably introducing variations into the measurement. Again, this procedure can be relatively cumbersome and the amount of trim removed can vary from sample to sample, introducing possible measurement error. Further, without careful control of the amount of vacuum pulled on the sample, significant variability can occur in the density measurement. Also, the speed at which the sample is raised to contact the plastic can cause the specimen or sample to puncture the sealing material allowing water to leak into the sample during liquid displacement testing. In addition, asphalt softens at approximately 150.degree. F. Heating the plastic can soften the asphalt layer at the surface and thereby change or alter the composition or condition of the sample during the water displacement test and also for any subsequent tests conducted subsequent to the liquid displacement density determination.
In recent years, the Federal Highway Administration ("FHA") has worked to improve the service life of bituminous pavements. As a result of a recent 5-year test, the FHA has recommended using compacted bituminous mixtures with larger aggregate size proportions. This larger aggregate size is believed to improve pavement performance and service life. Unfortunately, due to the use of larger aggregates, the asphalt specimens now prepared in the lab or extracted from the field have coarse and porous compositions. Typically, the coarse and porous compositions include a larger than 2% variation in air weight compared to saturated surface dry weight. This variation will make these samples unsuitable for the unsealed evaluation and, according to the ASTM standards discussed above, require that such samples be evaluated by sealed water displacement methods.